Spectroscopic analysis in a downhole environment sometimes makes use of a window or window surface total internal reflection to examine various sample materials. Both configurations may use instrumentation of substantial size, since the light sources used tend to be of low intensity, leading to the use of optics that concentrate light source output for greater resolution.
Windows have disadvantages when used in high-pressure environments, such as the difficulty of configuring the window mounting aperture to be pressure-tight, and keeping the window surface clean in the face of particle accumulation. Scattering via particles can greatly reduce the transmitted signal.
To reduce the overall size of the apparatus involved, windowed configurations may be designed to trade a reduced path length for reduced sensitivity. When total internal reflection is used, path lengths may be even more limited. For example, the sampling depth in a given material may be limited to approximately one-third of the wavelength of the light used in the analysis.
While these difficulties with performing spectroscopic analysis may be compounded when attempted in a downhole environment, it should be noted that many of these problems are not unique to the petroleum recovery industry. That is, they are common to many spectroscopic and photometric analysis designs, including those used for industrial in-line monitoring systems.